The complement system has emerged as a model of generation of biologically active products resulting from protein-protein interaction in plasma displaying marked structural versatility and proteolytic fragments generated from their precursor molecules. The significance of complement is, in part, emphasized by the fact that, the system is phylogenetically old. This application addresses three aspects of the relationship of structure to the function of the proteins of the complement system. 1. The phylogenetic evolution at DNA level of the regulatory proteins C4bp and H, utilizing data that indicates that lower vertebrates, including bony fish (sand bass), possess in their plasma a protein that serves the cofactor functions of H and C4bp in the degradation of human C4b and C3b. We will attempt to characterize the cDNA encoding for the sand bass protein and determine its nucleotide sequence. 2. The chemical definition of binding sites in C4b using monoclonal antibodies, enzymatically derived peptides, synthetic peptides, and polyclonal anti-peptide antibodies. In addition, the question of whether C4b exhibits a recognition function similar to that of C3b will be addressed using different activators, formation of the C4b2a enzyme and binding of the regulatory protein C4bp. 3. The elucidation of a unique pathway of activation of the alternative pathway that requires antibody, C1 and calcium ions, but is independent of C4 and C2.